diff --git a/pkgs/by-name/op/opensp/package.nix b/pkgs/by-name/op/opensp/package.nix
index d012bfa04f33..de0e48c4351e 100644
--- a/pkgs/by-name/op/opensp/package.nix
+++ b/pkgs/by-name/op/opensp/package.nix
@@ -7,6 +7,7 @@
   docbook_xml_dtd_412,
   docbook_xsl,
   autoconf,
+  autoconf269,
   automake,
   libtool,
   autoreconfHook,
@@ -61,7 +62,8 @@ stdenv.mkDerivation (finalAttrs: {
   ]
   # Clang 16 fails to build due to inappropriate definitions in the `config.h` generated by the
   # existing configure scripts. Regenerate them to make sure they detect its features correctly.
-  ++ lib.optional stdenv.cc.isClang autoreconfHook
+  # autoconf269: as autoconf 2.73 is trying to force C23 (-std=gnu23) which will fail, we use 2.69.
+  ++ lib.optionals stdenv.cc.isClang [ autoconf269 autoreconfHook ]
   ++ lib.optionals stdenv.hostPlatform.isCygwin [
     autoconf
     automake